This invention relates to the passivation of copper, aluminum, and other refractory metals. More particularly, it relates to a method and a structure produced by the method, for passivating a layer of copper on a substrate. It further relates to thin film transistors used in liquid crystal displays and the process of manufacturing the same.
There have been major difficulties in the use of copper as an electrical connection material in microelectronic circuits. These difficulties relate to the reactive nature of copper. Copper which has been deposited on a substrate will often react with subsequently deposited silicon containing materials and when such reaction occurs, it may delaminate or blister. This is especially significant in the production of liquid crystal displays.
Flat panel liquid crystal displays have been under development for well over a decade. At the present time they are used in laptop computers and other applications where it is desirable to have displays of low volume, low weight and low power consumption. However, various technological difficulties have hampered the production of flat panel liquid crystal displays of any great size.
Liquid crystal displays include a large number of picture elements or pixels arranged in a rectangular array. For example, in a large area liquid crystal display having high resolution, a matrix may be composed of 1280 columns and 1024 rows of pixels. In a color display, each pixel may have three subpixels for the primary colors, and thus there may be a total of nearly four million subpixels. In active matrix liquid crystal displays, each subpixel must be controlled by an active element, preferably a thin film transistor (TFT), which is constructed on a glass substrate.
The thin film transistors must each in turn be controlled by appropriate electronic circuitry which drives the display. In active matrix displays each thin film transistor is connected t o a gate control line (for a row of pixels) and a drain control line (for a column of pixels).
Liquid crystal displays used in such applications as portable televisions and laptop computers are generally illuminated by backlighting. A well-known problem is that only a small percentage (typically approximately three percent) of the light generated by the backlight gets through the liquid crystal display to the user. This is in part due to color filters associated with the pixels, but it is also due in large measure to the presence of the thin film transistors and the control lines extending from the edges of the panel to the thin film transistors. To the extent that the lines can be made more conductive, such as having gate lines made of highly conductive metals, the gate lines can be more narrow and a higher percentage of light may be transmitted through the liquid crystal panel.
As an alternative to using wider gate lines, which have the disadvantage noted above with respect to light transmission, it is possible to use thicker gate lines. However, thicker gate lines significantly increase the probability of producing so-called xe2x80x9ccrossover defectsxe2x80x9d during subsequent processing. In this processing the TFT structure is fabricated over the gate line, and the increased thickness leads to shorts or discontinuities that adversely affect the structure and therefore the operation of the TFT.
Materials which have been used for gate lines include molybdenum, chromium, and a molybdenum-tantalum alloy. While some success has been achieved, these materials are not sufficiently conductive. The short gate line pulses that are provided by the display driver chips located on the periphery of the liquid crystal display are attenuated due to the resistance and changed in shape in travelling from the edge of the display to its interior and the edge of the display opposite the driver chip. This gate line pulse distortion results in non-uniformity of display brightness, reduction of gray scale display capability (i.e. lack of contrast in some areas and therefore lack of uniformity in contrast across the display) and often produces noticeable flicker.
Until the present time, it has not generally been possible to use the most conductive material, copper, to form the gate lines to the thin film transistors on a liquid crystal display panel. This is because copper is very reactive with the subsequent layers of silicon dioxide or silicon nitride that must be placed over the portions of the copper gate lines which act as the gates of the TFT""s, In the case of silicon dioxide, delamination of the oxide film from the copper occurs. In the case of conventional silicon nitride the nitride film, and under certain conditions the copper, will blister. In addition, copper adhesion to glass substrates is often poor.
One solution to the copper adhesion problem is to increase the adhesion between copper and glass by using an adhesion layer such as chromium or titanium between the glass substrate and the copper line. However, this additional step increases cost, and does not address the main problem of reactivity and delamination when silicon dioxide or a conventional silicon nitride film is used over the copper to fabricate the gate insulator of the TFT.
One approach which directly addresses the copper reactivity problem is to deposit a copper line on the glass substrate and to encase or cap the copper line in another material, such as tantalum. Using this approach, a copper layer must be deposited and patterned using, for example, standard photoresist techniques. Then a layer of tantalum must be deposited and this layer must also be patterned. Those additional steps add considerable cost to the production process and increase the width of the gate line.
When an adhesion layer or a capping technique are used, the probability of crossover defects may also be increased due to the increased gate line thickness.
Thus, it would be highly advantageous to be able to form a liquid crystal display including thin film transistors having copper gate lines (and therefore copper gates) directly on a glass substrate without the need for an additional encapsulating protective metal such as Ta.
It is a principal object of the invention to provide a method for passivating copper, aluminum, or other refractory metals deposited on a substrate and a structure produced by the method.
It is another object of the invention to provide a liquid crystal display panel including thin film transistors which have copper deposited directly on a glass substrate.
It is still another object of the invention to provide a method of producing such liquid crystal displays.
It is a further object of the invention to provide a liquid crystal display including thin film transistors, and a method of making the same, wherein the copper gate has adequate adhesion to the glass layer without the use of an intermediate adhesion layer.
It is an additional object of the invention to provide a liquid crystal display including thin film transistors and a method of making the same, wherein copper may be deposited without a metal capping layer.
It is yet another object of the present invention to provide a structure and method for connecting copper gate lines to aluminum conductors.
In broad terms, the invention comprises the process of depositing a layer of ammonia-free silicon nitride over a layer of copper, aluminum, or another refractory metal deposited on a substrate. The invention also provides a three layer structure which comprises a substrate, a metal layer deposited on the substrate, and a passivating layer of ammonia-free silicon nitride on the metal layer. The structure may have a fourth layer over the passivating layer with the latter acting to prevent reaction of the copper with the fourth layer.
In accordance with the invention, in a liquid crystal display including a thin film transistor constructed on a glass substrate wherein the thin film transistor has a gate, a source and a drain, and a gate insulator between the gate and an active silicon layer, the improvement comprises the gate being copper deposited directly on the substrate and a layer of ammonia-free silicon nitride disposed between the copper gate and the subsequent insulating and active layers. Also in accordance with the invention, the ammonia-free silicon nitride layer also extends to portions of the substrate adjacent the copper gate.
The invention also contemplates a process for constructing a liquid crystal display, and in particular the thin film transistors therein on a substrate. Thin film transistors include a source, a gate, a drain and a gate insulator between the gate and an active silicon layer. In accordance with the invention, the improvement comprises depositing copper, aluminum, or another refractory metal directly on the substrate to form the gate and depositing a layer of ammonia-free silicon nitride between the gate and the gate insulator. Also in accordance with the process, the layer of ammonia-free silicon nitride extends to cover portions of the substrate adjacent the gate.
Also in accordance with the invention, in the case of copper gate, a structure and process for electrically connecting aluminum data metal (source/drain connections) to copper gate metal is provided, which uses a conduction material as a bridge, such as indium oxide or indium tin oxide. In accordance with the process, the conductive material which is preferably a layer of indium tin oxide is deposited on a portion of a substrate. A layer of copper is then deposited, so that it is in contact with a portion of the layer of conductive material, and a layer of metallization made of aluminum and/or molybdenum is then deposited, so that it contacts the conductive material. Prior to depositing the metallization, it is preferable to deposit a passivating layer over the copper, and to etch a via hole in the passivating layer to reach the conductive material, so that when the metallization layer is deposited, it extends through the via hole and makes an electrical connection to the conductive material. The passivating layer is ammonia-free silicon nitride, in accordance with the invention.